Normal adult hemoglobin comprises four globin proteins, two of which are alpha (α) proteins and two of which are beta (β) proteins. During mammalian fetal development, particularly in humans, the fetus produces fetal hemoglobin, which comprises two γ-globin proteins, instead of the two β-globin proteins. During fetal development or infancy, depending on the particular species and individual, a globin switch occurs, referred to as the “fetal switch”, at which point, erythrocytes in the fetus switch from making predominantly γ-globin to making predominantly β-globin. The developmental switch from production of predominantly fetal hemoglobin or HbF (α2γ2) to production of adult hemoglobin or HbA (α2β2) begins at about 28 to 34 weeks of gestation and continues shortly after birth until HbA becomes predominant. This switch results primarily from decreased transcription of the γ-globin genes and increased transcription of β-globin genes.
Abnormalities in hemoglobulin protein and function are associated with hemoglobinopathies. Hemoglobinopathies encompass a number of anemias of genetic origin in which there is a decreased production and/or increased destruction (hemolysis) of red blood cells (RBCs). These also include genetic defects that result in the production of abnormal hemoglobins with a concomitant impaired ability to maintain oxygen concentration. Some such disorders involve the failure to produce normal β-globin in sufficient amounts, while others involve the failure to produce normal β-globin entirely. The disorders associated with abnormalities in the β-globin protein are referred to generally as β-hemoglobinopathies. For example, β-thalassemias result from a partial or complete defect in the expression of the β-globin gene, leading to deficient or absent HbA. Sickle cell anemia results from a point mutation in the β-globin structural gene, leading to the production of an abnormal (sickled) hemoglobin (HbS). HbS RBCs are more fragile than normal RBCs and undergo hemolysis more readily, leading eventually to anemia (Atweh, (2001) Semin. Hematol. 38(4):367-73).
Kruppel-like factor 1 (erythroid) (KLF1) is a transcription factor involved in regulating adult globin expression. KLF1 is expressed in erythroid cells and positively regulates the adult β-globin gene. KLF1 is also an important activator of BCL11A, which encodes a suppressor of fetal hemoglobin. Loss-of-function mutations in the KLF1 gene have been associated with hereditary persistence of fetal hemoglobin (HPFH), which is characterized by persistent high levels of HbF in adults. The developmental switch from human fetal (γ) to adult (β) hemoglobin represents a clinically important example of developmental gene regulation. The transcription factor BCL11A is a central mediator of the γ-globin silencing and hemoglobin switching that occurs in erythroid progenitor cells. KLF1 has been suggested to control globin gene switching by directly activating β-globin and indirectly repressing γ-globin gene expression. Knockdown of KLF1 in human and mouse adult erythroid progenitors markedly reduced BCL11A levels and increased human γ-globin/β-globin expression ratios (Zhou et al. (2010) Nat. Genet. 42:742-744).
The need exists for identifying novel therapeutics that modulate one or more of: fetal switch, activation of fetal hemoglobin (HbF), thus altering globin chain levels, e.g., the ratio of γ-globin to β-globin. Such therapeutics can be used to treat subjects with a variety of abnormalities in hemoglobulin protein and function, such as hemoglobinopathies.